Conventional steam turbines, for example, the Worthington single stage steam turbines, may use a trip cup (alternatively, referred to as a trip disk) having a throw out arm for shutting off a steam source supplying steam to the steam turbine when the steam turbine speed (generally measured in revolutions per minute (RPM)) exceeds a certain predetermined value, and a mechanical flyweight style governor for varying (increasing or decreasing) an amount of steam supplied from the steam source to the steam turbine to adjust (increase or decrease) the speed of the steam turbine as long as the predetermined value is not exceeded. The predetermined value indicates that the steam turbine is operating at overspeed. Overspeed refers to a condition in which the steam turbine runs at a speed beyond its design limit. For instance, the steam turbine may overspeed when there is no or little load while power is applied or when the governor malfunctions.
FIG. 1 illustrates a cross-sectional overview of a conventional steam turbine 100 including a trip cup 102 and a mechanical flyweight style governor 104. The trip cup 102 and the mechanical flyweight style governor 104 are enclosed in a housing 108. The trip cup 102 is connected to the shaft 110 of the steam turbine 100 and rotates with the shaft 110. The mechanical flyweight style governor 104 is connected to the shaft 110 and is also connected to a steam source 116 (FIG. 1C) via a system of governor linkages 106. The mechanical flyweight style governor 104 moves the governor linkages 106 based on the speed of the steam turbine 100. The motion of the governor linkages 106 varies the steam supplied from the steam source 116 to the steam turbine 100 and, in turn, adjusts the speed of the steam turbine 100. In this manner, the operating speed of the steam turbine 100 is maintained.
FIG. 1A illustrates an enlarged cross-sectional view of the trip cup 102 and the mechanical flyweight style governor 104 of the conventional steam turbine 100 of FIG. 1. The mechanical flyweight style governor 104 includes a coil spring 1041 surrounding the shaft 110 and two governor flyweights 1042 disposed adjacent the coil spring 1041. The coil spring 1041 is positioned between the two governor flyweights 1042 at an inner end (closer to the trip cup 102) of the coil spring 1041 and an outer collar 1043 at an outer end (farther from the trip cup 102) of the coil spring 1041. The two governor flyweights 1042 are disposed surrounding the shaft 110. The two governor flyweights 1042 are connected to the shaft 110 adjacent the inner end of the coil spring 1041. The two governor flyweights 1042 rotate with the shaft 110 and at the speed of the shaft 110. The outer collar 1043 is connected to the governor linkages 106 such that a compression of the coil spring 1041 moves the governor linkages 106. During operation, a centrifugal force acts on the two governor flyweights 1042 causing the two governor flyweights 1042 to move outward and away from the coil spring 1041. This action of the two governor flyweights 1042 compresses the coil spring 1041 and moves the governor linkages 106. The motion of the governor linkages 106 adjusts the amount of steam output from the steam source 116 to the steam turbine 100 to adjust the speed of the steam turbine 100.
FIG. 1B illustrates a cross-sectional view of the trip cup 102 taken along the line 1B-1B in FIG. 1A. The trip cup 102 defines an opening 1022 through which the shaft 110 of the steam turbine 100 extends. A partially drilled hole 1023 is defined by the body 1021 of the trip cup 102. The partially drilled hole 1023 extends radially inwards from an outer circumferential surface of the body 1021 toward the central axis of the trip cup 102. The partially drilled hole 1023 houses a weight spring 1024 disposed therein and connected to a first end 1027 of the throw out arm 1025. The throw out arm 1025 is located in a recess 1026 in the trip cup 102 and a second end 1028 of the throw out arm 1025 is connected to the trip cup 102. The tension in the weight spring 1024 is adjusted such that, when the rotational speed of the steam turbine 100 exceeds the predetermined value, the throw out arm 1025 flies out from the trip cup 102. This results in a tripping action, commonly referred to as a “trip,” wherein the throw out arm 1025 deflects a trip paddle 112 (FIG. 1D) disposed adjacent the trip cup 102. Due to the deflection of the trip paddle 112, trip linkages 1029 (FIG. 1D) in contact with the trip paddle 112 are actuated, for example, released. Releasing the trip linkages 1029 shuts off the steam supplied to the steam turbine 100, thereby shutting off the steam turbine 100. In this manner, the steam turbine 100 is prevented from operating at overspeed.
FIG. 1C is a perspective view of the conventional steam turbine 100 of FIG. 1. Illustrated in FIG. 1 are the housing 108, the governor linkages 106, and the steam source 116. FIG. 1D is a top plan view of the trip cup 102 and the mechanical flyweight style governor 104 of the conventional steam turbine 100 of FIG. 1 contained in the housing 108. Also shown are the partially drilled hole 1023, the recess 1026, trip paddle 112, and the trip linkages 1029 connected to the steam source 116.
It has been found by those of ordinary skill in the art that the trip cup 102 may present a number of drawbacks. For example, the weight spring 1024 attached to the throw out arm 1025 may exhibit erratic behavior. As a result, sometimes the trip occurs prematurely, or sometimes the trip occurs too late. In addition, the throw out arm 1025 may break, thereby requiring replacement.
What is needed, therefore, is a steam turbine overspeed control system that provides reliable and efficient operation, requires low maintenance with ease of assembly and disassembly, and fits in the footprint of the existing trip cup.